Cryogenic fluids, such as liquid nitrogen, have been used successfully in a number of low-temperature applications such as for freezing food or other materials. Such cryogenic fluids are also often used for controlling the temperature of a process fluid such as those used in many chilling, cooling, or refrigeration systems. Such temperature control is typically provided by extracting heat from the process fluid to provide a cooling function. This type of temperature control is often used to maintain a process fluid at a constant predetermined temperature.
Using cryogenic fluids for controlling the temperature of many process fluids is challenging. This is because many process fluids have a freezing temperature far above the temperature at which typical cryogenic fluids are used. For example, a common process fluid used in industrial chillers contains water and ethylene glycol. Depending on the ratio of water to ethylene glycol, the freezing temperature of a water/ethylene glycol process fluid will be between about 0 degrees Celsius and minus 50 degrees Celsius. Liquid nitrogen is often used at a temperature of minus 195 degrees Celsius or lower, which is its boiling temperature. Cryogenic fluids can thus provide high heat transfer rates for cooling such process fluids because of the large temperature difference between the cryogenic fluid and a process fluid. However, because of such high heat transfer rates, undesirable freezing of a process fluid is possible when using cryogenic fluids to cool a process fluid. The process fluid can freeze onto internal surfaces of the heat exchanger thereby reducing the flow rate of the process fluid. Moreover, because cryogenic fluids provide such high heat transfer rates, it can be difficult to use cryogenic fluids to control a rate of temperature change in a process fluid or to maintain a steady predetermined temperature for an extended period of time.
One conventional approach used to address the above noted freezing problem is to use a heat exchanger that includes a heat transfer surface attached to a tube. The heat transfer surface is in contact with the tube and can thermally conduct heat away from the tube. For example a bank of cooling fins directly attached to the tube are often used. A process fluid flows through the tube and transfers heat to the heat transfer surface. Liquid nitrogen is poured or sprayed on the heat transfer surface of the heat exchanger to remove heat from the heat transfer surface. One problem encountered with this approach is that ice can build up on the heat transfer surface because moisture in the ambient air will condense and freeze on the liquid nitrogen cooled surface. When ice starts to grow and propagate, the heat transfer surface loses its thermal conductivity. The result is that the heat exchanger loses its heat transfer capacity rapidly or the process fluid in the tube freezes or both. The heat exchanger must then be defrosted before it can be put back to service.
Another known approach for addressing the freezing problem is to mix liquid nitrogen with room temperature nitrogen gas to reduce the driving force of the liquid nitrogen and provide a cryogenic gas with a warmer temperature. However, most of the latent heat of vaporization of the liquid nitrogen is lost in the mixing process and the heat transfer capability of the resulting cryogenic gas is reduced. Although this approach can help to avoid freezing of a process fluid, the rate of liquid nitrogen consumption is significantly increased and may be too high to be economically acceptable.
Yet another approach is to use one or more additional heat transfer fluids with lower freezing points to buffer the effect of the liquid nitrogen. That is, the liquid nitrogen is used to cool a heat transfer fluid (which may be used to cool another heat transfer fluid) and the heat transfer fluid is then used to cool the process fluid. Such an approach can be used to prolong the time it takes for the process fluid to freeze but given enough time, the process fluid may ultimately freeze. Moreover, this approach also adds substantial complexity and cost to a process for controlling the temperature of a process fluid.